Exploring New Tree Cover Gain and Net Change on Global Forest Watch

Earlier this year, researchers at the University of Maryland (UMD) published the first results of a new data set on tree cover change between 2000 and 2020, providing an updated look at tree cover height, tree cover gain, and net change in tree cover worldwide. The data shows that though the world lost more tree cover than it gained over the past two decades, 36 countries experienced a net gain in tree cover.

This data is now available on Global Forest Watch — read on to learn more about how to access it and how it differs from data you may have used in the past.

What’s New on GFW, and How Can I Access It?

Three different updated and new data sets on the GFW platform use the new data:

1. Tree cover height in 2000 and 2020: This data shows the height in meters of tree canopy cover in each 30-meter pixel. Users can set the minimum height threshold to view the extent of tree cover above a specific height. This layer replaces prototype data on tree cover height in 2019.

2. Tree cover gain between 2000 and 2020: This data shows areas that had tree cover at least five meters tall in 2020 but not in 2000. Tree cover gain can include regeneration of natural forests, regrowth after harvesting in tree plantation rotations, or restoration activities. This layer replaces previous data on tree cover gain between 2000 and 2012.

In addition to viewing the data on the map, users can access statistics on tree cover gain between 2000 and 2020 on the analysis pane and dashboards.

3. Net change in tree cover between 2000 and 2020: This data shows how much more or less tree cover a given location (country, state/province, or county/municipality) had in 2020 compared to 2000. This is a new data layer on GFW.

In addition to viewing the data on the map, users can access statistics on net change in tree cover between 2000 and 2020 on the analysis pane and dashboards.

Annual data for all three of these data sets will eventually be available. Read more about what we’re learning from these three data sets in the updated Forest Extent and Forest Gain Indicators in the Global Forest Review.

Why is GFW Moving from Canopy Cover to Tree Cover Height?

The first iteration of the tree cover change data on GFW, including the current annual tree cover loss data and the previous tree cover gain between 2000 and 2012, used definitions based on the percent canopy cover. The three updated and new data sets are all based instead on tree cover height, and eventually, annual updates will be based on height as well.

The main reason for this switch is because the new tree cover height data is more accurate. NASA’s GEDI mission, which uses a lidar sensor aboard the International Space Station, provides point-based measurements of vegetation structure, including forest canopy height, which UMD uses as training data to model tree height globally. The new data defining tree cover in 2000 using a minimum height of five meters resulted in an accuracy of 97%, compared to 95% accuracy when tree cover is defined as a minimum canopy cover of 30%.

Additionally, tree cover height allows for improved detection of tree cover gain. Unlike tree canopy cover, which reaches its maximum soon after tree growth begins, tree height changes gradually and is linearly related to forest biomass and carbon storage.

What’s Different About the New Tree Cover Gain Data?

Like the previous tree cover gain data, the new data shows gain between two points in time. However, the new data shows tree cover gain between 2000 and 2020, where the previous layer showed gain between 2000 and 2012. With eight additional years of data, we are now able to see a more recent view of gain that has occurred since the start of the millennium.

The old and new data also feature slightly different definitions of gain due to the methodological shift from measuring canopy cover to measuring tree cover height, as described above. The old data defined gain as an increase in tree cover to at least 50% canopy cover, while the new data defines gain as an increase in tree cover to at least five meters tall.

Like the previous data, the new gain data is available at 30-meter resolution and was produced by the GLAD lab at the University of Maryland based on Landsat satellite images. Future versions of this data will show annual tree cover gain, not just for two points in time.

What is Net Change, and Why is it Important?

The new net tree cover change data tells us how much more or less tree cover there was in 2020 than in 2000 in a particular administrative area such as a country, province or district. This is different from the current annual tree cover loss data or the updated tree cover gain data, both of which focus on gross changes in tree cover. The net change corresponds to the area of gross gain minus the area of gross loss to show the overall change (which can be negative or positive).

Net change is important because loss and gain data by themselves show just one side of the coin. If we consider loss on its own, we‘re not accounting for tree cover that grows back due to restoration efforts or natural regeneration (e.g. after a fire). If we consider gain on its own, we’re focusing only on new forest growth while turning a blind eye to any forests that are cut down next door.

Looking at net change in addition to gross gains and losses gives us a more holistic picture. Net change data is also important for better estimates of carbon fluxes from forests, and aligns better with other data such as the FAO’s “forest area net change” statistics in the Forest Resources Assessment.

However, net change alone doesn’t tell us about the quality of tree cover that was lost or gained. For example, primary forests in Southeast Asia that have been replaced by rubber plantations are considered to have no net change if that location had tree cover in both 2000 and 2020. It is important to look at other data sets available on GFW that can help us better understand these dynamics, such as those on primary forests, Intact Forest Landscapes and tree plantations.

How Does the Annual Tree Cover Loss Data Fit In?

Updates to the annual tree cover loss data will eventually also be based on tree cover height data to allow direct comparisons with the updated tree cover height and gain layers going forward. Watch for more information on this change and what it means for tree cover loss resources on GFW and in the Global Forest Review.

Previous years of loss data will also be eventually reprocessed. However, note that until then, the gain and loss data cannot be compared to each other because they are based on different methods and definitions.

What Are the Plans for Updating This Data?

GFW is working with UMD to deliver annual data on tree height, including tree cover loss and gain based on this data, which will eventually replace the 20-year data we have now. This will allow us to provide annual updates on global tree cover gain and net change in addition to our annual tree cover loss updates.

In the meantime, you can explore key takeaways about 2000 and 2020 extent and gain in the Global Forest Review, and check out the new data on the GFW map and dashboards.